The overall objectives of this project are to obtain the data required to facilitate explanation of the control of expression and the mechanism of action of the platelet components which mediate the aggregation of thrombin-activated platelets. Our previous work has shown that thrombin activation of platelets causes the expression of an endogenous platelet "lectin" not expressed by non-activated platelets. On the basis of a variety of observations, we have concluded that this thrombin-enhanced "lectin" is one of a number of mechanisms for mediating platelet aggregation. The expression of the thrombin enhanced "lectin" is secretion-dependent and caused by secreted materials, including proteins. Our objective is to isolate and characterize the secreted "lectin".